The present invention relates to a lubricating grease composition and in particular to a lubricating grease composition for ball bearings.
As the grease for bearings, in particular, ball bearings conventionally used in spindle motors for, for instance, hard disks of computers and recording devices such as CD-R""s, there has been used, for instance, those comprising a base oil such as dioctyl sebacate or a pentaerythritol ester; and a thickening agent such as lithium stearate or lithium hydroxy stearate. However, the acoustic life of the bearing has recently become insufficient because of the improvement of bearing precision due to an increase in the recording density and because of the requirement for a higher use temperature of such devices due to an increase in the number of revolution.
Accordingly, it is an object of the present invention to provide a lubricating grease composition whose frictional coefficient is lower than those observed for conventional grease compositions and which can ensure a long acoustic life when applied to bearings and, in particular, to a lubricating grease composition suitably used for ball bearings.
The inventors of this invention have conducted various studies to solve the foregoing problems associated with the conventional grease compositions for bearings, have found that it is effective to use a specific carbonate ester compound as a part or the whole of the base oil component in a lubricating grease composition which comprises a base oil and a thickening agent and have thus completed the present invention based on this finding.
According to the present invention, there is provided a lubricating grease composition which comprises a base oil and a thickening agent, wherein the base oil comprises 50 to 100% by weight of a carbonate ester compound represented by the following general formula (I):
R1Oxe2x80x94COxe2x80x94OR2xe2x80x83xe2x80x83(I)
wherein R1 and R2 each independently represents a saturated or unsaturated, linear or branched alkyl group having 6 to 30 carbon atoms, on the basis of the total weight of the base oil.
In preferred embodiments of the grease composition of the present invention, the thickening agent is preferably at least one member selected from the group consisting of lithium soaps, sodium soaps, calcium soaps, aluminum soaps, complex soaps thereof and urea compounds.
The lubricating grease composition of the present invention is suitably used as a lubricating grease composition for ball bearings of, in particular, spindle motors.
The carbonate ester compound represented by Formula (I) and used as a part or the whole of the base oil component according to the present invention is preferably those represented by Formula (I) wherein the substituents R1 and R2 each independently represents a saturated or unsaturated, branched alkyl group having 7 to 25 carbon atoms. Examples of preferred branched alkyl groups include those represented by the formula: xe2x80x94CH2CHR3R4. In this Formula, R3 represents a saturated linear alkyl group having 1 to 7 carbon atoms and R4 represents a saturated linear alkyl group having 4 to 12 carbon atoms. Particularly preferred are those represented by Formula (I) wherein the total carbon atom number of R3 and R4 ranges from 11 to 13. In the lubricating grease composition of the present invention, the carbonate ester compound of Formula (I) is used in an amount ranging from 50 to 100% by weight, preferably 60 to 100% by weight and more preferably 70 to 100% by weight based on the total weight of the base oil.
As the base oil which can be used in combination with the carbonate ester compound of Formula (I), there may be listed, for instance, ester type synthetic oils such as polyol ester oils, ether type synthetic oils such as alkyl diphenyl ethers, synthetic hydrocarbon oils such as poly-xcex1-olefins, mineral oils such as paraffinic mineral oils.
The thickening agent used in the present invention is preferably selected from the group consisting of lithium soaps, sodium soaps, calcium soaps, aluminum soaps, complex soaps thereof and urea compounds. Particularly preferred are lithium soaps such as lithium stearate and lithium 12-hydroxystearate. Examples of urea type thickening agents are, for instance, aliphatic or alicyclic diurea compounds and polyurea compounds.
The amount of the thickening agent used in the grease composition preferably ranges from 3 to 30% by weight and more preferably 5 to 25% by weight based on the total weight of the grease composition.
The grease composition of the present invention may further comprise additives commonly used in this field such as an antioxidant, a rust proof agent and/or an anticorrosive agent, in addition to the foregoing essential components.
As has been described above in detail, the lubricating grease composition of the present invention can not only ensure a frictional coefficient substantially lower than those expected by the use of the conventional grease compositions, but also ensure a substantially improved acoustic life, because of the use of a novel base oil component, i.e., a carbonate ester compound of Formula (I).